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The first step to perception is paying attention.  Selective attention , sometimes labeled the  cocktail party effect , is the ability to focus our concentration on specific stimuli. We mask the chaos of surrounding sensations and focus our attention on what we interpret to be important. In a classroom setting, we block out the sounds of whirring air conditioners, toe-tapping classmates, toilets flushing across the hallway, and other sensory distractions to focus on the most important source of information, the teacher. However, selective attention is often taken for granted. Children with developmental disorders like autism and Asperger’s syndrome, do not focus their attention on what we would generally define as the most important feature in a scene. Individuals with selective attention impairments practice this ability as part of therapeutic rehabilitation.

Depth Perception

Depth perception  is the ability to see in three dimensions and perceive distance from ourselves. A series of cues allows us to perceive depth. These signals are grouped as either binocular cues requiring two eyes or monocular cues, affecting each eye separately.

Binocular Cues

Having two eyes provides  binocular cues  that make it much easier to perceive depth.

iagram of eyes looking at letters A and B. The images of the two let Eye image: © 2010 Jupiterimages Corporation

Retinal Disparity

Because our eyes are set apart at a fixed angle, the image of a singular object falls on slightly different areas of each retina. This retinal disparity provides a clue to the relative depth of the object; the larger the disparity of images on the retinas, the closer the object.


oung girl staring at popsicle close to her face, with eyes crossed. © 2010 Jupiterimages Corporation

Another binocular cue is the  convergence  of the eyes as they rotate inward to follow an object as it moves closer. Hold your finger away from your body and slowly bring it in towards your nose. The eyes rotate inward to track your finger as it moves closer, triggering cues to the brain regarding the distance of the object.

Monocular Cues

Linear Perspective

hotograph of a road disappearing into the distance. © 2010 Jupiterimages Corporation

Some depth perception clues only require one eye. When parallel lines appear to converge, we perceive this as a measure of distance.

Who is furthest away from you in the image on the left? The size of the people is a cue to depth. Smaller people are further away and larger people are close.


wo playing cards, with the rightmost card laying on top of the leftmost card. Image: © 2010 Jupiterimages Corporation

Which of these cards is closer to you? The principle of interposition explains that we perceive an object as closer when it obstructs the view of another object. The card on the left appears closer because it is blocking the card on the right.

Atmospheric Perspective

ark trail, with three benches, winding into the distance. Image: © 2010 Jupiterimages Corporation

According to the  atmospheric perspective , objects at a distance appear hazy compared to closer objects. The closer bench is sharper and more focused, while the benches in the distance appear fuzzier as the distance increases.

Texture Gradient

rick patio surface, close up, fading into distance. Image: © 2010 Jupiterimages Corporation

We perceive depth as the texture of a scene becomes smaller. In this image, we can perceive distance because the shapes and sizes of the circular grid pattern create a  texture gradient , triggering our depth perception.

Motion Parallax

If we focus our gaze on an object as we are moving in a car, the objects behind our focal point appear to move slowly and objects in front move quickly. Our brains use this  motion parallax  to calculate the relative distances of the objects.

Top Down vs. Bottom Up Processing

As you read these words, what are you paying attention to? Are you analyzing each individual letter’s shape, building each letter into a word? If we used this  bottom-up processing , or building individual elements into a whole, reading a simple paragraph would take an eternity.

We can perceive words with only a few letters based on our predictions and expectations. This  top-down processing  allows us to perceive a whole group of sensations and break them into individual features.

Perceptual Sets

As we have seen, the same stimuli can be interpreted in many different perceptions. People’s expectations influence their subjective perceptual experiences; people see what they want to see rather than what is really there.

For an incredible example, look up a news story about some mysterious sightings of a Virgin Mary at a California chocolate factory.

From spaceships made out of clouds to an image of Elvis Presley’s face in toast,  perceptual sets  skew sensory reality into biased perception.

Size Constancy

hoto of man in desert. Looks like his foot is smashing a smaller man, who is act © 2010 iStockphoto

We know that an object’s size remains constant and that differences in size are interpreted as differences in depth. With one eye closed, focus your vision on a person across the room. Hold your hand in front of your gaze as if you were going to pinch that person’s head. Did their head magically shrink to a couple inches, or do objects at a distance appear smaller than closer objects?

Shape Constancy

iagram showing 4 stages of door opening. Closed, slightly open, near Image: © 2010 FLVS

We know that a door is usually the shape of a rectangle. However, if you follow the shape of a door as it opens, the door actually takes on the shape of a trapezoid and morphs into a much thinner rectangle. Our brain relies on shape constancy to perceive the door as a singular uniform shape.

Brightness Constancy

wo squares. One dark gray and one light gray. Both contain a smaller square that is Image: Public domain, via Wikimedia Commons

Which of the center squares is darker? Most people respond that the center square on the left side is darker, when in fact both center squares are exactly the same brightness. The surrounding larger squares confuse our visual perception systems.

Color Constancy

lue mug with shadow falling across half of it. Image: © 2010 Jupiterimages Corporation

What color is this cup? The most common answer is blue, but if you observe closely, the cup is not a uniform color at all. The shadows darken various features, and the light source highlights others. From previous experiences, we know cups are usually painted a solid color, so due to  color constancy , we perceive the cup as a single color. Our minds perceive the color fluctuations as the result of changes in illumination, not deliberate changes in actual color of the cup.

Figure and Ground

You probably have seen Federal Express trucks rushing to deliver packages, but have you ever noticed the arrow shape in between the “E” and “x” letters? If not, go take at the log on their web site, or you may even have a package or letter laying around. See if you can spot the arrow.

This is a reversible image describing the Gestalt principle of figure and ground. People typically divide their visual field into the figure, or focal point of their attention, and the background (ground) surrounding the main image. The figure/ground principle reinforces that the single stimulus can prompt multiple perceptions.


identical vertical lines in groups of two. © 2010 FLVS

Would you describe this image as four skinny columns or three wide columns? People often report seeing the slimmer columns due to the Gestalt principle of  proximity . We group things together that are close together.


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